fix: update with_a2a_extensions to append instead of overwriting

Author: ishymko

src/a2a/client/service_parameters.py

@@ -1,7 +1,10 @@
 from collections.abc import Callable
 from typing import TypeAlias
 
-from a2a.extensions.common import HTTP_EXTENSION_HEADER
+from a2a.extensions.common import (
+    HTTP_EXTENSION_HEADER,
+    get_requested_extensions,
+)
 
 
 ServiceParameters: TypeAlias = dict[str, str]
@@ -46,15 +49,26 @@ def create_from(
 def with_a2a_extensions(extensions: list[str]) -> ServiceParametersUpdate:
     """Create a ServiceParametersUpdate that adds A2A extensions.
 
+    Merges the supplied URIs with any extensions already present in the
+    A2A-Extensions service parameter, deduplicating and producing a stable
+    (sorted) order. Calling this multiple times in a chain accumulates the
+    requested extensions instead of overwriting prior values.
+
     Args:
-        extensions: List of extension strings.
+        extensions: List of extension URIs to advertise.
 
     Returns:
         A function that updates ServiceParameters with the extensions header.
     """
 
     def update(parameters: ServiceParameters) -> None:
-        if extensions:
-            parameters[HTTP_EXTENSION_HEADER] = ','.join(extensions)
+        if not extensions:
+            return
+        existing = parameters.get(HTTP_EXTENSION_HEADER)
+        merged = sorted(
+            get_requested_extensions([existing] if existing else [])
+            | set(extensions)
+        )
+        parameters[HTTP_EXTENSION_HEADER] = ','.join(merged)
 
     return update

tests/client/test_service_parameters.py

@@ -0,0 +1,78 @@
+"""Tests for a2a.client.service_parameters module."""
+
+from a2a.client.service_parameters import (
+    ServiceParametersFactory,
+    with_a2a_extensions,
+)
+from a2a.extensions.common import HTTP_EXTENSION_HEADER
+
+
+def test_with_a2a_extensions_sets_header_when_empty():
+    """First call on empty parameters sets the joined URIs."""
+    parameters = ServiceParametersFactory.create(
+        [with_a2a_extensions(['ext-b', 'ext-a'])]
+    )
+
+    assert parameters[HTTP_EXTENSION_HEADER] == 'ext-a,ext-b'
+
+
+def test_with_a2a_extensions_merges_disjoint_calls():
+    """A second call with disjoint URIs unions both sets."""
+    parameters = ServiceParametersFactory.create(
+        [
+            with_a2a_extensions(['ext-a']),
+            with_a2a_extensions(['ext-b']),
+        ]
+    )
+
+    assert parameters[HTTP_EXTENSION_HEADER] == 'ext-a,ext-b'
+
+
+def test_with_a2a_extensions_deduplicates_overlapping():
+    """Overlapping URIs do not produce duplicates."""
+    parameters = ServiceParametersFactory.create(
+        [
+            with_a2a_extensions(['ext-a', 'ext-b']),
+            with_a2a_extensions(['ext-b', 'ext-c']),
+        ]
+    )
+
+    assert parameters[HTTP_EXTENSION_HEADER] == 'ext-a,ext-b,ext-c'
+
+
+def test_with_a2a_extensions_empty_is_noop():
+    """Calling with an empty list leaves any existing header untouched."""
+    parameters = ServiceParametersFactory.create(
+        [
+            with_a2a_extensions(['ext-a']),
+            with_a2a_extensions([]),
+        ]
+    )
+
+    assert parameters[HTTP_EXTENSION_HEADER] == 'ext-a'
+
+
+def test_with_a2a_extensions_empty_does_not_create_header():
+    """Calling with an empty list on empty parameters adds nothing."""
+    parameters = ServiceParametersFactory.create([with_a2a_extensions([])])
+
+    assert HTTP_EXTENSION_HEADER not in parameters
+
+
+def test_with_a2a_extensions_output_is_sorted():
+    """Output ordering is deterministic (sorted) regardless of input order."""
+    parameters = ServiceParametersFactory.create(
+        [with_a2a_extensions(['c', 'a', 'b'])]
+    )
+
+    assert parameters[HTTP_EXTENSION_HEADER] == 'a,b,c'
+
+
+def test_with_a2a_extensions_merges_existing_header_value():
+    """Existing comma-separated header values are parsed and merged."""
+    base = ServiceParametersFactory.create_from(
+        {HTTP_EXTENSION_HEADER: 'ext-a, ext-b'},
+        [with_a2a_extensions(['ext-c'])],
+    )
+
+    assert base[HTTP_EXTENSION_HEADER] == 'ext-a,ext-b,ext-c'